The present invention relates generally to systems and techniques for controlling fueling of an internal combustion engine, and more specifically to systems controlling engine fueling in accordance with information relating to vehicle location. A further aspect of the invention concerns a system and method for preventing an overspeed condition in a turbocharger when the engine is operating at higher altitude, lower ambient air pressure conditions.
It is presently known in the internal combustion engine industry, and particularly in the medium and heavy duty truck industry, to select engine fueling strategies based on presumed geographic conditions, wherein the geographic conditions are presumed from certain engine and/or vehicle operational parameters. In one known system, engine acceleration is monitored, presumed geographic conditions are determined therefrom and an appropriate one of a pair of fueling strategies is selected based upon the presumed geographic conditions. For example, during periods of continuous accelerations (i.e. stop and go traffic), the vehicle is presumed to be operating in or near a city, and a low emissions engine fueling map is selected for operation in accordance therewith. Conversely, during periods of steady state engine operation, the vehicle is presumed to be operating on an open highway, and a fuel economic engine fueling map is selected for operation in accordance therewith.
While engine fueling control systems of the foregoing type have been somewhat helpful in reducing emissions in areas designated by the Environmental Protection Agency (EPA) as non-attainment areas, they have several drawbacks associated therewith. For example, due to the engine acceleration-based determination of presumed geographic conditions, the foregoing system will typically select the fuel economic engine fueling map when driving on city freeways and beltways, thereby increasing vehicle emissions in or near low emissions urban areas.
As another example drawback, engine fueling control systems of the foregoing type typically switch to the fuel economic engine fueling map only after prolonged periods of steady state engine operation. Thus, during stops in rural areas, such as at weigh stations and toll booths, such systems typically revert to the low emissions fueling map. The same result occurs when the vehicle is in operation and the vehicle operator is required to interrupt steady state engine operation, such as when downshifting to negotiate a steep grade or when slowing down the vehicle in construction areas. In either case, fuel economy unnecessarily deteriorates.
As yet another drawback, known engine fueling control systems of the foregoing type provide for selection between only a fuel economic or a low emissions engine fueling map. However, either engine fueling map may be undesirable, or even counterproductive, under certain conditions requiring increased engine output (either via engine output power or engine output torque), such as when climbing steep grades. Increased engine output under such conditions would be advantageous in several respects. For example, vehicle operators would be grateful for increased engine output when driving through mountainous regions, and such increased output would reduce the need to down-shift, thereby reducing wear and tear on vehicle components. Moreover, such increased output would likely decrease transit time and allow vehicle operators to pass similarly rated vehicles while still maintaining good fuel economy. Further, vehicle purchasers could purchase lower rated engines and still get higher engine output when needed. The lower rated engines would resultantly last longer than the higher rated predecessor engines, and customer satisfaction would likely correspondingly increase.
In some cases, a vehicle engine is provided with a turbocharger that increases the intake airflow to the engine. In a typical turbocharger, a turbine is driven by the engine exhaust. The turbine is linked to a compressor section so that the compressor rotates as the turbine is driven. The compressor section draws in and compresses ambient air, with the compressor output being fed to the engine air intake manifold.
The rotational speed of the turbocharger is directly related to the flow rate of the engine exhaust, which is a function of engine speed. Since the turbocharger draws in ambient air, the speed of the device is also dependent upon the pressure of the air being drawn into the compressor section. For a given engine speed, the rotational speed of the turbocharger will increase as the pressure of the ambient air decreases.
As with any rotating machinery, the turbocharger has a maximum operational speed. Exceeding this speed can lead to failure, often catastrophic, of the turbocharger as the yield strength of the rapidly rotating components is exceeded. Ordinarily this characteristic of turbochargers does not pose a problem since the turbocharger is calibrated to withstand operation at the maximum engine operating power at altitudes below 10,000 ft.
However, problems arise when turbocharged engines are operated at high altitudes, where the ambient air pressure is less than at the sea level calibration pressure. At higher elevations, and consequently lower pressures, the turbocharger speed can exceed its limit speed when the engine is operating at or near its maximum rated power. Some known engine controllers rely upon an air pressure sensor mounted in the vehicle to determine the ambient air pressure. When the ambient pressure drops below a certain value, fueling to the engine is reduced, reducing the engine power or speed, and ultimately reducing the speed of the turbocharger.
One problem with this approach is that most on-board pressure sensors are accurate to only .+-.5 percent. An error of this magnitude is equivalent to an error of .+-.1000 ft. in the altitude of the vehicle. This sensor error can result in premature derating of the engine performance if the sensor reading is less than the actual ambient pressure. On the other hand, if the sensor reads a pressure that is greater than the true ambient pressure, the risk of turbocharger overspeed arises.
In addition to the accuracy problems associated with on-board pressure sensors, a further issue concerns the cost of the instrument. The typical pressure sensor can cost in the range of $10-20. Over an entire fleet of vehicles and through an expected number of sensor replacements, the overall cost of an on-board pressure sensor can become very high. Moreover, as with any engine-based sensor, ambient pressure sensors deteriorate and fail over time, which again yields a risk of turbocharger overspeed.
What is therefore needed is a system for controlling engine fueling, which overcomes the drawbacks of known engine fueling control systems. Ideally, such a system should control engine fueling based on actual (or somewhat accurately estimated) vehicle location/position. Such a system could dramatically reduce emissions in low emissions area and more accurately enable an appropriate engine fueling map regardless of the states, trends or statuses of engine/vehicle operational parameters. Such a system should further make available not only fuel economic and low emissions engine fueling maps, but should further provide for one or more higher output engine fueling maps to assist vehicle operators in hilly or mountainous regions.
A system is also needed that can accurately determine the ambient pressure for use in controlling the engine, and ultimately a turbocharger associated with the engine. Ideally, such a system would not require additional hardware or instruments. Moreover, the need extends to a system that is essentially immune to independent failure.